1. Field of the Invention
This invention relates to a heat transfer recording method and apparatus for transferring the ink of an ink sheet to a recording medium to thereby accomplish image recording on the recording medium.
The term "heat transfer recording apparatus" covers, for example, a facsimile apparatus, an electronic typewriter, a copying apparatus, a printer apparatus, etc.
2. Related Background Art
Generally, in a heat transfer printer, use is made of an ink sheet having heat-meltable (or heat-sublimating) ink applied to a base film, and the ink sheet is selectively heated by a thermal head correspondingly to an image signal and the melted (or sublimated) ink is transferred to recording paper to thereby accomplish image recording. Generally such ink sheet is one from which the ink is completely transferred to the recording paper by one time of image recording (so-called one time sheet) and therefore, it has been necessary that after the termination of recording of one character or one line, the ink sheet be conveyed by an amount corresponding to the length of the record and then the unused portion of the ink sheet be reliably brought to the position for recording. Therefore, the quantity of ink sheets used is increased and as compared with an ordinary thermosensitive printer for recording on thermosensitive paper, the running cost of the heat transfer printer tends to become high.
In order to solve such a problem, there have been proposed heat transfer printers wherein recording paper and an ink sheet are conveyed with a speed difference therebetween as seen in U.S. Pat. No. 4,456,392, Japanese Laid-Open Patent Application No. 58-201686, Japanese Patent Publication No. 62-58917. As described in these publications, an ink sheet capable of effecting plural times of image recording (so-called multiprint sheet) is known, and if such ink sheet is used, when a record length L is to be continuously recorded, the conveyed length of the ink sheet conveyed after or during each recording cycle can be made smaller than the length L (L/n:n&gt;1) to thereby accomplish recording. Thus, the use efficiency of the ink sheet becomes n times as great as that before and a reduction in the running cost of a heat transfer printer can be expected. This recording system will hereinafter be referred to as multiprint.
However, in the multiprint using such an ink sheet, as in the conventional heat transfer printer, as seen in the aforementioned publications, the conveying directions of the recording paper and the ink sheet have been the same. This relation, if expressed in a mathematic expression, is V.sub.P =n.multidot.V.sub.I, where V.sub.P represents the velocity of the recording paper relative to the thermal head, and V.sub.I represents the velocity of the ink sheet relative to the thermal head.
In contrast, as a result of our experiment, we have found that a higher relative velocity of the recording paper and the ink sheet is more advantageous when multiprint is carried out in the heat transfer system. This will hereinafter be described.
In the conventional heat transfer system, it has been necessary that the ink of the ink sheet be completely peeled from the base film by one cycle of heating, while in the multiprint system, the ink is recorded by n cycles of heating and therefore it is necessary that approximately 1/n of the ink layer be separated and transfer-recorded by one cycle of heating. On the other hand, the ink layer in the ink sheet is, for example, heat-meltable and therefore if the time from after the ink sheet is heated by the thermal head until the ink layer is peeled becomes long, the shearing force required to shear and separate the ink layer will become great. Therefore, if the time from after the thermal head is heated until the ink is transferred becomes long, it will become difficult to separate the ink layer properly (the unit of 1/n) and transfer it to the recording paper. Thus, unless the relative velocity of the recording paper and the ink sheet is high to a certain degree, the ink layer in the ink sheet cannot be separated well.
The conventional relative velocity V.sub.PI of the recording paper and the ink sheet, if V.sub.P =n.multidot.V.sub.I, is EQU V.sub.PI =V.sub.P -V.sub.I =V.sub.P -V.sub.P /n=(1-1/n)V.sub.P ( 1)
and when the ink sheet is at rest, the relative velocity V.sub.PI becomes equal to V.sub.P, but the relative velocity V.sub.PI when the ink sheet is moved cannot become higher than the conveyance velocity V.sub.P of the recording paper. Also, the above-mentioned equation shows that the conveyance velocity V.sub.P of the recording paper cannot be made very high, for example, a sufficient relative velocity cannot be obtained in the case of a line printer or the like, and when the conveyance velocity V.sub.P of the recording paper is small, there has been the tendency that separation of the ink in the ink sheet cannot be accomplished well and the recording paper is pulled by the conveyance velocity of the ink sheet or the conveyance velocity of the ink sheet is pulled by the conveyance velocity of the recording paper.